In blow molding processes, molten resins must form into stable parisons for a time long enough to permit a mold to enclose the parison. If these molten polymers do not possess sufficient melt strength or melt viscosity, the extruded tube will tend to elongate or draw under its own weight so that the resulting blow molded article will have non-uniform wall thicknesses, low surface gloss, and poorly defined sample shape.
Polymers such as polyesters, polyamides, polyethers, and polyamines when melted, generally form thin liquids having low melt viscosities. These low melt viscosity materials are unsuited or are only poorly suited for the manufacture of extruded shapes, tubes, deep-drawn articles, and large blow molded articles. In order to overcome this disadvantage and to convert these polymers to a form better suited for the above-mentioned manufacturing techniques, it is known to add compounds to the plastics which will increase their melt viscosities. The materials which are added to increase the melt viscosity of the plastics are generally cross linking agents, as described, for example, in U.S. Pat. No. 3,378,532. Such cross linking agents may be added during the condensation reaction by which the plastics are formed, and/or to the plastics after their formation (prior to, or during their melting). Examples of cross linking agents which may be added to the plastics after their formation and before or after their melting in order to increase the melt viscosity include compounds containing at least two epoxy or isocyanate groups in the molecule, organic phosphorus compounds, peroxides, bishaloalkylaryl compounds, and polyesters of carbonic acid.
these known cross linking agents which are added to increase the melt viscosity of the polymer are not completely satisfactory. They may, for instance, cause an excessively rapid and large increase in viscosity or form reaction products which have an adverse influence on the quality of the plastics. Furthermore, the results obtained with the use of these known cross linking agents are not always uniform or reproducible. For example, when polyesters of carbonic acid are used to increase the melt viscosity, the degree of viscosity increase is generally dependent not only upon the amount of additive used but also upon its molecular weight and on the state of the polycondensation reaction at which the addition takes place.
It has been observed that besides having sufficient melt viscosity or melt strength, polymers which are to be used in blow molding and related applications should also possess s sufficient die swell, i.e., the molten polymer should expand as it is released from the extrusion die. This die swell is important for blow molding applications since (a) the larger the diameter of the extruded polymer, the easier it is for air to be blown into the melt, and (b) the greater the die swell, the greater the expansion of the molten polymer to fit the particular mold.
It has been further observed that polyesters having low intrinsic viscosities are particularly difficult to blow mold and are also unsuited for many other related applications.
It has been still further observed that polymers which are utilized in blow molding applications should also possess a high degree of shear sensitivity, i.e., the molten polymer should thin out and become less viscous upon the application of increasingly higher rates of shear.
A shear sensitive polymer is more easily processed through an extruder and aids in providing an efficient blow molding process. Thus, a polymer having the combination of enhanced intrinsic viscosity and shear sensitivity may be readily extruded (while being subject to a high degree of shear) and thereafter evidence a high degree of stability in the parison as a result of the high intrinsic viscosity and reduced shear present therein.
Thus, although the prior art illustrates the use of numerous additives to modify various properties of polyesters, the search has continued for improved processes for improving certain rheological properties of polyesters, particularly, the intrinsic viscosity and melt strength such that these polyesters may be useful in blow molding and related applications.
Carbodiimide additives have been used to stabilize polyesters (see, e.g., U.S. Pat. Nos. 3,193,522; 3,193,523 and 3,193,524). For example, in U.S. Pat. No. 3,193,522, there is provided a process for stabilizing the polyester compounds against hydrolytic degradation by intermixing therewith polycarbodiimide additives having molecular weights of at least about 500 and having more than three carbodiimide groups in the molecules. Such intermixing is insufficient to provide a reaction of the type discussed herein and therefore incapable of achieving the improved properties obtainable by the practice of the presently claimed invention. Also, U.S. Pat. Nos. 3,193,523 and 3,193,524 discloses the use of monocarbodiimides to stabilize polyesters. However, none of these prior attempts to stabilize polyesters by adding carbodiimide additives have produced polyesters having improved melt strength intrinsic viscosity die swell, and shear sensitivity, characteristics such that they may be capable of use in extrusion and related applications.
Still further, U.S. Pat. No. 2,284,896 discloses a process for reacting inter alia a carbodiimide containing compound with an organic substance having a plurality of groups containing a reactive hydrogen. None of the reactive hydrogen containing compounds, however, are disclosed as being polyesters.